It is known from German Auslegeschrift (Published Pat. Application No.) 2,353,960 to operate presses with two-stage pressure buildup, in order to shape at a lower pressure a rough-pressed body initially, prior to extrusion, without requiring a special rough press for the purpose.
For extruding plastic or elastic material, e.g., in extruding presses, it is known to raise the pressure very slowly at first after filling the pressing cylinder with the loose material to be molded, until a certain pre-extrusion pressure is reached in the material. Despite subjecting the material in the pressing cylinder to a vacuum, it has not been possible as a rule to prevent air inclusions from remaining; these air inclusions are then compressed quasi-adiabatically by an excessively rapid pressure increase caused by the forward advance of the plunger, and when the molded material emerges from the pressing cylinder, such inclusions destroy the molded body as a result of a sudden release of stress.
In order to avoid quasi-adiabatic compression phenomena of the type described, it is conventional, at least in the initial phase of the pressure buildup in the molding cylinder, to control manually a valve which influences the inflowing hydraulic fluid in such manner that initially only a very slow pressure rise occurs in the pressure cylinder.